1. Field of the Invention
The present invention relates to an ion attachment mass spectrometry apparatus, more particularly relates to an ion attachment mass spectrometry apparatus suitable for measurement of components of sample gases to be measured.
2. Description of the Related Art
An ion attachment mass spectrometry (IAMS) apparatus is an apparatus for ionizing component molecules of a sample gas for mass spectrometry without disassociating the sample gas into component atoms, ions, atomic groups or other fragments. Such an apparatus is particularly effective for analysis of easily disassociating organic matter. The conventional ion attachment mass spectrometry apparatuses are reported by Hodge (“Analytical Chemistry”, 1976, vol. 48, no. 6, p. 825), Bombick (“Analytical Chemistry”, 1984, vol. 56, no. 3, p. 396), Fujii (“Analytical Chemistry”, 1986, vol. 61, no. 9, p. 1026), and Fujii (“Chemical Physics Letters”, 1992, vol. 191, no. 1.2, p. 162).
As patent publications disclosing the related arts, there are Japanese Patent Publication (A) No. 6-11485, Japanese Patent Publication (A) No. 2001-174437, Japanese Patent Publication (A) No. 2001-351567, Japanese Patent Publication (A) No. 2001-351568, Japanese Patent Publication (A) No. 2002-124208, and Japanese Patent Publication (A) No. 2002-170518.
The ion attachment mass spectrometry apparatus is provided with a metal ion emitter, an attachment region, and a mass spectrometer. These are arranged from the upstream side to downstream side in the ion attachment mass spectrometry apparatus in the order of the metal ion emitter, attachment region, and mass spectrometer. The metal ion emitter, attachment region, and mass spectrometer are all provided in a vacuum atmosphere of less than atmospheric pressure. The metal oxide of the metal ion emitter is heated to emit Li+ or other positively charged metal ions. When the sample gas is introduced into the attachment region, the metal ions gently attach at locations with polarity of the molecules of the sample gas, that is, at locations with a bias in charge. The molecules to which the metal ions attached become ions having a positive charge (hereinafter referred to as “attached ions”) overall. In the attached ions, the surplus energy, that is, the energy becoming a surplus at the time of attachment, is extremely small, so the molecules will not disassociate. Further, the surplus energy in the attached ions is quickly stripped by the collision with ambient gas such as N2, so the attached ions become stable. The attached ions are transported from the attachment region to the mass spectrometer under the force of the electric field. The attached ions are then classified by mass and measured by the mass spectrometer.
When the above sample gases are obtained from the atmospheric air, auto emissions or the like, they contain large amounts of H2O (water component) in the form of vapor (or steam) derived from humidity in addition to the gas to be actually measured. In many cases, for example, several percent of H2O in terms of partial pressure (absolute humidity) is included in the case of room temperature such as atmospheric air, and 10% of H2O is included in the case of a high temperature such as auto emissions. Therefore, when measuring these sample gases by the above ion attachment mass spectrometry apparatus, there is a higher concentration of H2O in the attachment region than even the gas to be primarily measured.
Since H2O has a high polarity, that is, a strong bias in charge, it easily attaches to the metal ions. Therefore, if there is a large amount of H2O in the attachment region, the metal ions will attach to the H2O and the majority of the metal ions will be used up. As a result, the metal ions attached to the molecules of the sample gas to be inherently measured will be reduced and the measurement sensitivity of the sample gas will drop sharply.
Further, as to H2O, polymers in which a single metal ion is attached to a plurality of molecules are also easily produced. For example, in case that a monomer such as H2O Li+ would be normally produced as result, instead, a dimer of (H2O)2Li+, trimer of (H2O)3Li+, quatramer of (H2O)4Li+ and the like will be produced. Further, H2ON2Li+ or other polymers will be produced by being bonded with the N2 that exists in large quantities as the ambient. These will overlap with the peaks of the sample gas and will end up causing interference. Therefore, the inherent reliability of measurement of the sample gas drops sharply. The above situation becomes a major cause of deterioration of the measurement performance of the ion attachment mass spectrometry apparatus.
Note that there is also the method of using a desiccant, cooler, etc. in order to remove the H2O and dehydrate the sample gas. This method, however, often also ends up simultaneously removing the gas to be measured and therefore is not practical.